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Home -> Community -> Usenet -> comp.databases.theory -> Re: Need help on Functions Dependency problems, Urgent!!!
lordsofring wrote:
>I'm a newbie on function dependency problem, currently working on the
>following questions, but havn't got a clue. Anyone can solve this will
>be highly apprieciated.
Of course, but if we give you the answers you won't learn anything. So tell us what you don't understand and we will try to give you hints. For now I will give some very short hints:
>
>F = {A->B, C->D, AC->E, B->E, CE->A, BD->C} is a set of function
>dependencies on S (ABCDEF)
>
>1. Find all the candidate keys of S regarding to F.
Hint1: There is a rule that you can use to find smaller superkeys:
If K is a superkey and X->A a fun. dep. that holds (with X a set of attributes and A single attribute) then you can remove A from K if X is a subset of K.
Hint2: ABCDEF is the mother of all (super)keys
>2. Find a cover for F+|ABCD, F+ is the closure of F.
Hint: Just do what it says, determine F+|ABCD and then find a minimal cover for it. But try to be smart about it because it may otherwise be a lot of work.
>3. Find a BCNG and loss less join decomopsetion for S. explain the
>procedure
Depends a bit on what your instructor has told you. The usual way:
- determine minimal cover
- split off dependencies that violate BCNF
Another possiblity:
- determine minimal cover
- give every fd its own relation
>4. Does the BCNF and loss less join decomposition in (3) preserve the
>set F of fd's?
Hint: the definition of preserving F in decomposition D is that all fd's in F follow from the restrictions of F+ to the relations in D.
Good luck,